library: replace return types of constructors with Self

[xtqqczze]

Fix some instances of clippy::use_self

[rustbot]

r? @Mark-Simulacrum

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• Owners of files modified in this PR: @ChrisDenton, libs
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[Mark-Simulacrum]

Can you say more about the motivation for this change? It seems like unnecessary churn to me.

[xtqqczze]

According to clippy::use_self it is: "Unnecessary repetition. Mixed use of Self and struct name feels inconsistent."

[Mark-Simulacrum]

There's a reason that rule is allow-by-default even in clippy. I don't think this is worth the churn to run that lint. Thanks!